1. Field of the Invention
The present disclosure relates to an object information acquisition apparatus, a display method, and a storage medium. In particular, the present disclosure relates to a technique for transmitting elastic waves to an object, and displaying distribution information acquired by receiving reflected waves from the object.
2. Description of the Related Art
In an ultrasonograph which is an object information acquisition apparatus, the spatial resolution in the depth direction in image data formation based on the pulse echo method can be generally represented by (nλ)/2, where λ indicates the wavelength of ultrasonic waves and n indicates the number of transmission waves. For example, when the ultrasonograph transmits ultrasonic waves having a center frequency of 12 MHz for two wavelengths, the spatial resolution is about 0.13 mm.
The pulse echo method will be described below. When the ultrasonograph transmits ultrasonic pulses (elastic waves) to an object, the ultrasonic waves are reflected by the object according to the acoustic impedance difference between tissues inside the object, and return to the ultrasonograph. Then, the ultrasonograph receives the reflected waves and generates image data by using received signals of the reflected waves. Typically, the ultrasonograph applies delay and sum to the received signals, acquires an envelope, and converts the envelope into luminance values to generate image data. Repeating ultrasonic wave transmission and reception in a plurality of directions or positions inside the object enables acquiring luminance information on a plurality of scanning lines in the directions in which ultrasonic wave transmission and reception were made. Arranging the luminance information on the plurality of scanning lines enables imaging an inside of the object.
It is common that the ultrasonograph adds a temporal deviation to received signal waveforms between elements by using a plurality of conversion elements for converting the ultrasonic wave into an electrical signal for focusing inside the object both for transmission and reception.
On the other hand, applying adaptive signal processing, which has developed in the field of the radar, together with ultrasonic waves enables improving the spatial resolution. M. SASSO et al., Medical Ultrasound Imaging Using The Fully Adaptive Beamformer, Proc. Acoustics and Speech Signal Process. volume. 2, pp. 489-492 (March 2005 discusses a technique using the Capon method (adaptive signal processing) to improve the spatial resolution in the direction perpendicular to the depth direction (direction perpendicular to the scanning line direction).
As a technique for improving the spatial resolution in the depth direction (scanning line direction), Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina and Toru Sato: Conf Proc IEEE Eng Med Biol Soc. 2010; 1: 5298-5301 discusses results of imaging of the layer structure of the blood vessel wall by applying the Frequency Domain Interferometry (FDI) method and the Capon method (adaptive signal processing). Applying the FDI method and the Capon method to received signals enables improving the spatial resolution in the depth direction. However, it is assumed that a plurality of reflective layers exists in a signal range (within a processing range) in the depth direction clipped for the FDI processing. A plurality of reflected waves from close reflective layers is highly likely to have high mutual correlations. It is known that applying adaptive signal processing, such as the Capon method, to received signals of a plurality of reflected waves having such high correlations will cause an unexpected operation, such as negating a desired signal. By using the frequency averaging technique to reduce (suppress) the effect caused by signals (correlated interference waves) having such correlations, the FDI method and the Capon method are applicable to the received signals of the reflected waves.
Applying adaptive signal processing, such as a method combining the FDI and Capon methods, enables improving the spatial resolution of an image. However, if an image generated by such a new technique is displayed, a user (particularly, a doctor) may feel odd since the user is familiar with the conventional B mode image (an image produced by applying delay and sum to a plurality of received signals to acquire an envelope, and converting the envelope into luminance values). In particular, if only an image generated through adaptive signal processing is displayed, the odd feeling may increase.